The present invention relates to the production and processing of high Tc superconducting bismuth-strontium-calcium-copper-oxide materials.
Since the discovery of the copper oxide ceramic superconductors, their physical and chemical properties have been widely studied and described in many publications, too numerous to be listed individually. These materials have superconducting transition temperatures (Tc) greater than the boiling temperature (77xc2x0 K.) of liquid nitrogen. However, in order to be useful for the majority of applications, substantially single phase superconducting materials with high critical current densities (Jc) are needed. In general, this requires that the grains of the superconductor be crystallographically aligned, or textured, and well sintered together. Several members of the bismuth-strontium-calcium-copper-oxide family (BSCCO), in particular, Bi2Sr2CaCu2O8 (BSCCO 2212) and Bi2Sr2Ca2Cu3O10 (BSCCO 2223) have yielded promising results, particularly when the bismuth is partially substituted by dopants, such as lead ((Bi,Pb)SCCO).
Composites of superconducting materials and metals are often used to obtain better mechanical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by the well-known xe2x80x9cpowder-in-tubexe2x80x9d or xe2x80x9cPITxe2x80x9d process which includes, for multifilamentary articles, the three stages of: forming a powder of superconductor precursor material (precursor powder formation stage); filling a noble metal billet with the precursor powder, longitudinally deforming and annealing it, forming a bundle of billets or of previously formed bundles, and longitudinally deforming and annealing the bundle to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material in a surrounding noble metal matrix (composite precursor fabrication stage); and subjecting the composite to successive asymmetric deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing stage). General information about the PIT method described above and processing of the oxide superconductors is provided by Sandhage et al., in JOM, Vol. 43, No. 3 (1991) pages 21 through 25, and the references cited therein, by Tenbrink, Wilhelm, Heine and Krauth, Development of Technical High-Tc Superconductor Wires and Tapes, Paper MF-1, Applied Superconductivity Conference, Chicago (Aug. 23-28, 1992), and Motowidlo, Galinski, Hoehn, Jr. and by Haldar, Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors, paper presented at Materials research Society Meeting, Apr. 12-15, 1993.
In the composite precursor fabrication stage, longitudinal deformation operations, i.e., wire drawing and/or extrusion, which form the billet or bundle into an elongated shape such as a wire or tape are followed by low temperature anneals, typically on the order of 200xc2x0 C. to 450xc2x0 C. at 1 atm in air for silver, to relieve strain energy introduced by deformation, without causing substantial reaction of the precursor powder or melting or grain growth in the silver. FIG. 1 (prior art) is a typical annealing curve showing silver hardness as a function of annealing temperature. In some instances, a high temperature thermal anneal, typically on the order of 600xc2x0 C. at 1 atm in air for silver, has been performed prior to the first bundle deformation step in the stage to bond the billets to one another. In other instances, where high strain deformations involving reductions of 100% or more have been performed, a high temperature thermal anneal, typically on the order of 600xc2x0 C. at 1 atm in air for silver, has been included as the last step in the stage in order to relieve the strain energy in the matrix material prior to thermomechanical processing.
The deformation portions of the deformation and annealing cycles in the thermomechanical processing stage, are asymmetric deformations which create alignment of precursor grains in the core (xe2x80x9ctexturedxe2x80x9d grains) which facilitate the growth of well-aligned and sintered grains of the desired superconducting material during later thermal processing stages. Examples are rolling and the isostatic pressing cycle described in U.S. patent application Ser. No. 07/906,843 (US ""843) filed Jun. 30, 1992 entitled xe2x80x9cHigh Tc Superconductor and Method for Making Itxe2x80x9d, which is herein incorporated in its entirety by reference. They may be followed by anneals to relieve strain energy in the metal portions of the composite precursor. A series of heat treatments is also typically performed during the thermomechanical processing stage to promote powder reactions, including final thermomechanical treatment stages employed to more fully convert the filaments to the desired final, highly textured superconducting phase, preferably BSCCO or (Bi,Pb)SCCO 2223. The thermomechanical processing may be carried out by any conventional method, such as for example those described in Sandhage et al, supra, Tenbrink et al, supra, Haldar, supra, and in U.S. Pat. No. 5,635,456 issued Jun. 3, 1997, entitled xe2x80x9cImproved Processing for Oxide Superconductors,xe2x80x9d and U.S. Pat. No. 5,661,114 issued Aug. 26, 1997, entitled xe2x80x9cImproved Processing of Oxide Superconductorsxe2x80x9d, and U.S. patent application Ser. Nos. 08/468,089, (US ""089) filed Jun. 6, 1995, entitled xe2x80x9cImproved Deformation Process for Superconducting Ceramic Composite Conductorsxe2x80x9d, now issued as U.S. Pat. No. 6,247,224, and 08/651,169 (US ""169) filed May 21, 1996, entitled xe2x80x9cA Novel reaction for High Performance (Bi,Pb)2Sr2Ca2Cu3Oy Compositesxe2x80x9d, now issued as U.S. Pat. No. 5,798,318, all of which are hereby incorporated in their entirety by reference.
The general process is practiced in several variants depending on the starting powders, which may be, for example, metal alloys having the same metal content as the desired superconducting core material in the xe2x80x9cmetallic precursorxe2x80x9d or xe2x80x9cMPITxe2x80x9d process, or mixtures of powders of the oxide components of the desired superconducting oxide core material or of a powder having the nominal composition of the desired superconducting oxide core material in the xe2x80x9coxide powderxe2x80x9d or xe2x80x9cOPITxe2x80x9d process. OPIT precursor powders may be prepared by reacting raw powders such as the corresponding oxides, oxalates, carbonates, nitrides or nitrates of the metallic elements of the desired superconducting oxide. One or more subsequent chemical reactions, some of which typically occur inside the formed filaments, create the superconducting material in combination with greater or lesser amounts of non-superconducting secondary phases. Because the desired superconducting material is formed by a series of chemical reactions, its performance will depend on the quality and chemical composition of the starting materials and on the subsequent processing conditions, such as temperature, time, and atmosphere. Different processing conditions will give rise to different phases or different ratios of phases, some of which, being easier to mechanically texture or more likely to achieve complete reaction into the final superconducting material, are more desirable than others. Various intermediate reactions may be deliberately promoted in order to create more desirable intermediate phases or to increase the ratio of the final superconducting material to the secondary phases in the finished product.
For example, it has been observed that the orthorhombic phase of BSCCO 2212 responds better to the asymmetric deformation required for deformation-induced texturing resulting in a denser, less porous oxide grain structure, and so, undergoes texturing to a much greater extent than the corresponding tetragonal phase. Moreover, the orthorhombic phase of (Bi,Pb)SCCO 2212 represents doping of lead into the BSCCO solid state structure with the concomitant conversion of the lead-free tetragonal phase into the orthorhombic phase. The lead-doped orthorhombic phase readily converts to the final superconductor (Bi,Pb)SCCO 2223 to give a high quality superconductor over a large temperature range. In comparison, the lead-free tetragonal BSCCO phase does not convert readily into (Bi,Pb)SCCO 2223. By controlling phase conversions, it is possible to make use of the advantages of the orthorhombic and tetragonal phases, by using the particular phase most suited to the operation to be performed. Methods of controlling the phase composition of the precursor powder during its preparation and during subsequent thermomechanical processing, are described, for example in U.S. patent application Ser. No. 08/467,033 (US ""033) filed Jun. 6, 1995, now issued as U.S. Pat. No. 5,942,466, and entitled xe2x80x9cProcessing of (Bi,Pb)SCCO Superconductor in Tapes and Wiresxe2x80x9d, which is herein incorporated in its entirety by reference. In the process described in US ""033, an elongated BSCCO superconducting article is manufactured by first heating a mixture of raw materials of a desired ratio of constituent metallic elements corresponding to a final superconducting BSCCO material at a first selected processing temperature in an inert atmosphere with a first selected oxygen partial pressure for a first selected time period. The first processing temperature and partial pressure are cooperatively selected to form a dominant amount of certain desired BSCCO precursor phases, preferably including a tetragonal BSCCO 2212 phase, along with the secondary phases necessary for the production of the desired final superconducting phases, in the reacted mixture. A composite article may be formed using this reacted precursor powder. substantially surrounded by a constraining metal matrix. Prior to the texture-inducing deformation operation, the article is subjected to a heat treatment at a second selected processing temperature in an inert atmosphere with a second selected oxygen partial pressure for a second selected time period which favors conversion of the tetragonal BSCCO phase into the corresponding orthorhombic BSCCO. 2212 phase, so as to form a dominant amount of an orthorhombic BSCCO 2212 phase in the reacted mixture. Thereafter, the multifilamentary article is textured by deformation and thermally processed into a BSCCO 2223 oxide superconductor article. Selection of appropriate processing conditions, for example as described in Luo et al., xe2x80x9cKinetics and Mechanism of the (Bi,Pb)2Sr2Ca2Cu3O10 Formation Reaction in Silver-Sheathed Wires,xe2x80x9d Applied Superconductivity, Vol. 1, No. 1/2, pp. 101-107 (1993), will allow the BSCCO 2223 to substantially inherit the texture, whether orthorhombic or tetragonal, or its 2212 precursor phase.
Reference to the xe2x80x9corthorhombic phasexe2x80x9d and the xe2x80x9ctetragonal phasexe2x80x9d recognizes the existence of two crystallographic structures for BSCCO superconducting materials, the tetragonal and the orthorhombic structures. The tetragonal structure has equivalent a and b axes with a lattice parameter of about 5.4 angstroms. The conversion of the tetragonal to the orthorhombic structure corresponds to the formation of an oxygen deficient structure with a and b axes which are unequal in length. See, Jeremie et al in Supercond. Sci. Technol. 6 (1993) pages 730 through 735, which is herein incorporated by reference in its entirety. The conversion occurs simultaneously with the incorporation of a substituent having a variable oxidation state, i.e., Pb or Sb, into the BSCCO structure. Thus the formation of the orthorhombic phase is indicative of the reaction of the dopant carrier. The conversion is indicated by the broadening (and under some conditions, complete splitting) of the XRD 200 and 020 peaks at 33xc2x0(2xcex8).
As compared to certain prior art approaches, this process provides a method for preparing precursor powders having a controlled phase composition in a single step reaction process, and improved phase control during subsequent thermomechanical processing. However, it has been found that when the tetragonal to orthorhombic phase conversion is performed in multifilamentary composite precursors, processing inhomogeneities tend to occur and blister-like defects frequently form, both of which can adversely affect the Jc performance of the desired superconducting composite article. The inventors believe that during the composite precursor fabrication stage, the mechanical force applied to reduce the cross-section of the multifilamentary precursor will tend to work to a greater degree on the filaments in the outer portions of the precursor and cause an inhomogeneous stress distribution, both through the diameter of the precursor and along its length. Therefore, the outer filaments and their surrounding matrix material will deform more than those near the center of the precursor, creating a distribution of differently sized filaments. Further, the inhomogeneous stress distribution creates filament slippages, breaks and other defects in the filaments. During the composite precursor fabrication stage, the multifilamentary precursor also tends to absorb gas and moisture which becomes trapped, creating blisters, particularly in the filaments and at the interfaces between the filaments and the surrounding metal matrix. These problems are characteristic of PIT processes generally, but they are exacerbated during processes requiring high temperature treatments and oxygen release, such as the tetragonal to orthorhombic phase transformation. Significant amounts of oxygen must be released from the filaments during the formation of the oxygen-deficient structure which characterizes the orthorhombic phase, and removed by diffusion through the matrix material. If the cross-sections of the filaments and surrounding matrix material are non-uniform, the phase transformation cannot proceed uniformly and undesired phases will result. The positive pressure inside the blisters will tend to prevent oxygen release from the adjacent filaments causing additional inhomogeneities in the phase transformations. Moreover, during the high temperature phase conversion, the gas in the blisters will tend to expand and water and other condensed phases will volatize so the blisters will grow significantly in size, hindering subsequent processing steps.
It is desirable to provide a process which provides improved powder phase control coupled with improved oxygen control and defect management during tetragonal to orthorhombic phase conversions. It is also desirable to provide a superconducting composite article with reduced defect levels and improved Jc performance.
The present invention provides a means of controlling the phase composition of a precursor powder for the BSCCO superconducting materials, particularly Pb-doped BSCCO materials, with selected primary and secondary phases during those thermomechanical processing steps which occur inside the composite precursor, while minimizing formation of blisters and other defects in the composite. In general, the method includes the steps of consolidating the multifilamentary composite precursor during the composite precursor fabrication stage in order to promote grain growth of the constraining metal at the interfaces between said metal and said filaments and eliminate voids in the article, and subsequently performing one or more oxygen-releasing phase conversions inside the composite precursor under conditions of time, temperature and oxygen partial pressure cooperatively selected to promote the growth of desired powder phases while controlling the rate of oxygen release from the composite. A consolidation step is performed after at least the last rebundling step and consolidation is completed before any high strain longitudinal deformation of the bundle.
In one aspect of the present invention, an elongated BSCCO, (preferably Pb-doped BSCCO) superconducting article is manufactured by first heating a mixture of raw materials of a desired ratio of constituent metallic elements corresponding to a final superconducting BSCCO material at a first selected processing temperature in an inert atmosphere with a first selected oxygen partial pressure for a first selected time period. The first processing temperature and partial pressure are cooperatively selected to form a dominant amount of certain selected intermediate phases having a dominant amount of a tetragonal BSCCO phase in the reacted mixture. A billet is then formed which is comprised of the reacted mixture substantially surrounded by a constraining metal, and a bundle is then formed including a plurality of billets. Next, the bundle is thermomechanically consolidated by applying heat and pressure under conditions cooperatively selected to cause interdiffusion of the constraining metal at the interfaces between said metal and said filaments and substantially complete elimination of voids in the article, and consolidation is completed before any high strain longitudinal deformation is performed on the bundle. The article is then heated at a second selected processing temperature in an inert atmosphere with a second selected oxygen partial pressure for a second selected time period cooperatively selected to form a dominant amount of an orthorhombic BSCCO phase in the reacted mixture. A texture-inducing deformation is performed on the article to form an elongated precursor article of a desired texture. In a preferred embodiment, the elongated precursor article is then heated at a third selected processing temperature in an inert atmosphere with a third selected oxygen partial pressure for a third selected time period. The third processing temperature and the third oxygen partial pressure are cooperatively selected to convert at least a portion of the orthorhombic BSCCO phase to the final superconducting BSCCO material, and preferably to create a dominant about of the final superconducting BSCCO material.
In another aspect of the invention, a novel process for the production and processing of high quality, high Tc BSCCO superconductors, preferably Pb-doped BSCCO superconductors, starts with fabrication of a bundle including a plurality of billets, each billet containing at least one filament comprising a dominant amount of an tetragonal BSCCO phase with selected intermediate phases, and substantially surrounded by a constraining metal. Next, the bundle is thermomechanically consolidated to form a multifilamentary precursor article by applying heat and pressure under conditions cooperatively selected to cause interdiffusion of the constraining metal at the interfaces between said metal and said filaments and substantially complete elimination of voids in the article, and consolidation is completed before any high strain longitudinal deformation is performed on the bundle. The article is then heated at a second selected processing temperature in an inert atmosphere with a second selected oxygen partial pressure for a second selected time period, the second processing temperature, the second time period and the second oxygen partial pressure being cooperatively selected to form a dominant amount of an orthorhombic BSCCO phase in the reacted mixture. In preferred embodiments, a texture-inducing deformation is performed on the article to form an elongated precursor article of a desired texture; and the article is thereafter heated at a third selected processing temperature in an inert atmosphere with a third selected oxygen partial pressure for a third selected time period, all cooperatively selected to convert at least a portion of the orthorhombic BSCCO phase to the final superconducting BSCCO material, and preferably to create a dominant about of the final superconducting BSCCO material.
By xe2x80x9cfinal BSCCO superconducting materialxe2x80x9d, as that term is used herein, it is meant the chemical composition and solid state structure of the superconducting material after all processing of the precursor is completed. It is typically, though not always, the oxide superconductor phase having the highest Tc or Jc.
By xe2x80x9cdominant amountxe2x80x9d of a designated BSCCO phase, as that term is used herein, it is meant that the designated phase, such as a BSCCO-2223 phase, the orthorhombic BSCCO-2212 phase or the tetragonal BSCCO-2212 phase, is the dominant phase present in the precursor powder, as selected among the members of the homologous BSCCO series of oxide superconductor. A xe2x80x9cdominant amountxe2x80x9d includes more than 50 vol %, preferably more than 80 vol %, and most preferably, more than 95 vol % of the members of the homologous BSCCO series as the designated phase.
In a preferred embodiment, the final superconducting material includes a BSCCO-2223 phase. In another preferred embodiment, the final superconducting material includes a (Bi,Pb)SCCO-2223 phase. In another preferred embodiment, the dominant orthorhombic phase includes a BSCCO-2212 phase. In yet another preferred embodiment, the dominant orthorhombic phase includes a doped BSCCO-2212 phase, where the dopant substitutes for bismuth. The dopant may be, for example, lead (Pb) or antimony (Sb), and is preferably Pb.
The multifilamentary superconducting BSCCO article may be of any elongated shape or form. It is typically in tape or wire form as a constraining metal matrix surrounding a plurality of filaments, each comprising BSCCO powder. The metal is typically a noble metal or an alloy substantially comprising a noble metal. A noble metal is substantially inert to oxidation under conditions used in high temperature superconductor manufacture. By xe2x80x9calloyxe2x80x9d, as it is used herein is meant a solid solution or uniform dispersion of metals in one another, or a composite of a metal and a small amount of another substance. It may include oxide-dispersion strengthened (ODS) metals and alloys. Silver and silver alloys, including ODS silver, are preferred noble metals, while silver and high silver content alloys, containing at least 90% silver, are most preferred.
By xe2x80x9csubstantially complete elimination of voidsxe2x80x9d is meant that the average void fraction of closed pore space in the article, determined immediately after the consolidation step, is less than 5%, more preferably less than 3%, and most preferably less than 1%. The void fraction is calculated as the measured void space in the non-filament area of an article cross-section divided by the difference between the total non-filament area of the article cross-section.
By xe2x80x9chigh strain longitudinal deformationxe2x80x9d is meant a longitudinal deformation such as drawing or rolling to a total reduction in the area of the perpendicular cross-section greater than 67% in one or more passes. By xe2x80x9clow strain longitudinal deformationxe2x80x9d is meant a longitudinal defirmation such as drawing or rolling to a total reduction in the area of the perpendicular cross-section no greater than 67% in one or more passes. Since consolidation is completed before any high strain longitudinal deformation is performed on the bundle, longitudinal deformations performed on the bundle prior to and during the consolidation step (but after the completion of any prior consolidation step) may cumulatively reduce the perpendicular cross-sectional area of the bundle by no more than a total of about 67%.
In one aspect of the invention, the bundle is consolidated by simultaneously applying pressure and heat under conditions sufficient to substantially eliminate voids in the article without buckling the filaments, and to promote grain growth of the constraining metal. In a preferred embodiment, the bundle is consolidated by hot isostatic pressing in an inert gas, typically at a pressure in the range of about 3 atm. to about 999 atm., and a temperature in the range of about 200xc2x0 C. to about 750xc2x0 C. for a time in the range of about 1 hour to about 36 hours, where the matrix is silver or a high silver content alloy. Preferably, the bundle is hot isostatically pressed at a pressure in the range of about 3 atm. to about 420 atm., and a temperature in the range of about 200xc2x0 C. to about 600xc2x0 C., and most preferably, the bundle is hot isostatically pressed at a pressure in the range of about 3 atm. to about 140 atm., and a temperature in the range of about 300xc2x0 C. to about 600xc2x0 C.
In another aspect of the invention, the bundle is consolidated by applying one or more sequences of pressure followed immediately by heating to promote grain growth of the constraining metal. In a preferred embodiment of the invention, the heating step is a thermal anneal, typically performed at a pressure of about 1 atm. and a temperature in the range of about 400xc2x0 C. to about 750xc2x0 C. for a time in the range of about 5 minutes to about 50 hours where the matrix is silver or a high silver content alloy. A cold isostatic pressing step, under conditions sufficient to substantially eliminate voids in the article without buckling the filaments, is performed just before the thermal anneal. The article may be cold isostatically pressed in liquid pressurization media, air, or another gas at a pressure which is preferably in the range of about 10 atm. to about 2000 atm., and most preferably in the range of about 100 atm. to about 1100 atm., at about ambient temperature for a time in the range of about 5 minutes to about 100 hours.
In another preferred embodiment, the bundle is consolidated by one or more drawing sequences sufficient to substantially eliminate voids in the article without buckling the filaments, followed immediately by heating to promote grain growth in the constraining metal. The heating step is a thermal anneal, typically performed at a pressure of about 1 atm. and a temperature in the range of about 400xc2x0 C. to about 750xc2x0 C. for a time in the range of about 5 minutes to about 50 hours for silver or a high silver content alloy, and the drawing step is performed before the thermal anneal. In a preferred embodiment, the pressure step is drawing to a reduction such that the total area reduction of the bundle after it is formed but before the thermal anneal is substantially equal to the average void fraction of closed pore space in the bundle. In another preferred embodiment, the bundle is drawn to a total reduction in perpendicular cross-sectional area (including all other post-bundling, pre-consolidation reductions) of no more than 67%, and preferably between about 5% and about 50%, in 1 to 6 passes of about 5% to about 25% per pass, with optional intervening low temperature anneals, in the range of about 200xc2x0 C. to 450xc2x0 C. in air for 5 minutes to 2 hours, to reduce strain energy.
One or more iterations of the consolidation step may be performed on a bundle. In some embodiments, some or all of sealing, cleaning, evacuation or low strain deformation operations may be performed on the billets or bundles at any time before consolidation. If several bundling iterations are performed, a consolidation step may be performed after each bundling operation or once when the final bundle is assembled. In some embodiments, additional deformation and anneal cycles are performed on the article after it has been consolidated to further reduce the cross-section of the composite.
In a preferred embodiment, the second processing temperature, the second processing time and the second oxygen partial pressure are cooperatively selected such that their values are below the stability line defined by the minimum values at which the dominant BSCCO 2212 phase melts or decomposes, and above the stability line defined generally by the maximum values at which Cu2+ decomposes to Cu+. In a preferred embodiment, the dominant orthorhombic phase includes a Pb-doped BSCCO-2212 phase, the selected intermediate phases include Pb4+ phases, and the second processing temperature, the second processing time and the second oxygen partial pressure are also cooperatively selected such that their values fall within the region where substantial portions of the Pb+4 phases can be reduced to Pb+2 phases. The Pb+4 phases typically may include (Ca2xe2x88x92xSrx)PbO4 and (Ca2xe2x88x92xxe2x88x92ySrxCuy)(Pb1xe2x88x92nBin)Oz. In yet another preferred embodiment, the heating step includes maintaining the temperature of the mixture in a range of 650xc2x0 C. to 870xc2x0 C. and the oxygen partial pressure in a range of 1.0 atm O2 to 0.0001 atm O2, and preferably maintaining the temperature of the mixture in a range of 700xc2x0 C. to 860xc2x0 C. and the oxygen partial pressure in a range of 0.5 atm O2 to 0.04 atm O2, and most preferably maintaining the temperature of the mixture in a range of 740xc2x0 C. to 850xc2x0 C. and the oxygen partial pressure in a range of 0.21 atm O2 to 0.04 atm O2, for a time period of about 0.01 to about 10 hours.
In other preferred embodiments, the step of forming a dominant amount of a tetragonal BSCCO phase in the precursor powder is carried out at a first temperature in the range of 700-850xc2x0 C. and a first oxygen partial pressure in the range of 0.04 atm to 1 atm.
In preferred embodiments, the texture-inducing deforming step is an asymmetric deformation such as rolling, pressing extruding or drawing through an aspected die or twisting. By xe2x80x9casymmetric deformationxe2x80x9d, it is meant any deformation which provides a substantial change in aspect ratio or shear strain in the material. By xe2x80x9clongitudinal deformationxe2x80x9d is meant any deformation which provides substantial increase in the length and decrease in the perpendicular cross-sectional area of the article. In another embodiment of the present invention, the texture-inducing deforming step and final oxide superconductor-forming heating step are sequentially repeated.
In a preferred embodiment, the third processing temperature, the third processing time and the third oxygen partial pressure are cooperatively selected such that their values are below the stability line defined by the minimum values at which the desired final BSCCO superconductor, preferably BSCCO 2223 or (Bi, Pb)SCCO 2223, melts or decomposes, and above the stability line defined by the minimum values at which the dominant orthorhombic BSCCO phase melts or decomposes. In other preferred embodiments, the final conversion step may alternatively preferably include heating at a temperature in the range of 800xc2x0 C. to 845xc2x0 C. and at an oxygen pressure in the range of 0.003 to 0.21 atm O2 . It may alternatively preferably include heating in a first step in the range of about 810-850xc2x0 C.; heating in a second step in the range of about 780-840xc2x0 C.; and heating in a third step in the range of about 600-800xc2x0 C., said first, second and third heating steps at an oxygen pressure in the range of 0.003 to 0.21 atm O2. It may alternatively preferably include heating at a first temperature in the range of 650xc2x0 C. to 795xc2x0 C. and at a first oxygen pressure in the range of 0.0001 to 0.075 atm O2; and heating at a second temperature in the range of 800xc2x0 C. to 845xc2x0 C. and at a second oxygen pressure in the range of 0.003 to 0.21 atm O2. It may also alternatively include heating at a first temperature in the range of 650xc2x0 C. to 795xc2x0 C. and at a first oxygen pressure in the range of 0.0001 to 0.075 atm O2; and heating in a second step in the range of about 810-850xc2x0 C.; heating in a third step in the range of about 780-840xc2x0 C.; and heating in a fourth step in the range of about 600-800xc2x0 C., said second, third and fourth heating steps at an oxygen pressure in the range of 0.003 to 0.21 atm O2.
In yet another preferred embodiment, the final conversion step includes ramping through a temperature range and an oxygen partial pressure range, such that the temperature and oxygen partial pressure range cooperatively include a value at which the dominant orthorhombic BSCCO phase, preferably (Bi,Pb)SCCO 2212, decomposes. The ramping is at a rate sufficiently rapid such that the decomposition of the dominant orthorhombic BSCCO phase is kinetically disfavored. In a preferred embodiment, the ramp rate is greater than 0.1xc2x0 C./min and most preferably is in the range of 0.1 to 100xc2x0 C./min.
In another aspect of the present invention, a multifilamentary composite precursor for a superconducting article is provided comprising a constraining metal matrix surrounding a plurality of filaments each containing a dominant amount of a tetragonal BSCCO 2212 phase, characterized in that the filaments are substantially unbuckled and average void fraction of closed pore space in the precursor is less than 5%, preferably less than 3% and most preferably less than 1%.
In another aspect of the present invention, an elongated BSCCO superconducting article is manufactured by: first, fabricating a bundle including a plurality of billets, each billet comprising a constraining metal matrix surrounding one or more filaments comprising selected intermediate phases having a dominant amount of the tetragonal BSCCO phase. Next, the bundle is thermomechanically consolidated to form a multifilamentary precursor article by applying heat and pressure under conditions cooperatively selected to cause interdiffusion of the constraining metal at the interfaces between said metal and said filaments and substantially complete elimination of voids in the article, and consolidation is completed before any high strain longitudinal deformation is performed on the bundle. The article is heated at a second selected processing temperature in an inert atmosphere with a second selected oxygen partial pressure for a second selected time period, the second processing temperature, the second time period and the second oxygen partial pressure being cooperatively selected to form a dominant amount of an orthorhombic BSCCO phase in the reacted mixture. In preferred embodiments, a texture-inducing deformation is performed on the article to form an elongated precursor article of a desired texture; and the article is thereafter heated at a third selected processing temperature in an inert atmosphere with a third selected oxygen partial pressure for a third selected time period, all cooperatively selected to convert at least a portion of the orthorhombic BSCCO phase to the final superconducting BSCCO material.
In another aspect of the present invention, an elongated BSCCO superconducting article is manufactured by first heating a mixture of raw materials of a desired ratio of constituent metallic elements corresponding to a final superconducting BSCCO material at a first selected processing temperature in an inert atmosphere with a first selected oxygen partial pressure for a first selected time period. The first processing temperature and partial pressure are cooperatively selected to form a dominant amount of certain selected intermediate phases having a dominant amount of a tetragonal BSCCO phase in the reacted mixture. A billet is then formed which is comprised of the reacted mixture substantially surrounded by a constraining metal, and a bundle is then formed including a plurality of billets. Next, the bundle is thermomechanically consolidated by applying heat and pressure under conditions cooperatively selected to cause interdiffusion of the constraining metal at the interfaces between said metal and said filaments and substantially complete elimination of voids in the article, and consolidation is completed before any high strain longitudinal deformation is performed on the bundle. The article is heated at a second selected processing temperature in an inert atmosphere with a second selected oxygen partial pressure for a second selected time period cooperatively selected to form a dominant amount of an orthorhombic BSCCO phase in the reacted mixture. A texture-inducing deformation is performed on the article to form an elongated precursor article of a desired texture. In a preferred embodiment, the elongated precursor article is then heated at a third selected processing temperature in an inert atmosphere with a third selected oxygen partial pressure for a third selected time period. The third processing temperature and the third oxygen partial pressure are cooperatively selected to convert at least a portion of the orthorhombic BSCCO phase to the final superconducting BSCCO material.
In preferred embodiments, the final oxide superconductor comprises (Bi,Pb)SCCO 2223, the tetragonal BSCCO phase comprises tetragonal (Bi,Pb)SCCO 2212 and the orthorhombic BSCCO phase comprises orthorhombic (Bi,Pb)SCCO 2212. In other preferred embodiments, the final oxide superconductor comprises BSCCO 2223, the tetragonal BSCCO phase comprises tetragonal BSCCO 2212 and the orthorhombic BSCCO phase comprises orthorhombic BSCCO 2212.